Change GitHub workflow runner.

Runner was set to macos-latest (synonym for macos-14) which currently
has 3CPUs (M1), 7GB RAM and 14GB SSD. The build was timing out after
6 hours. Changing to macos-13 configuration which has 4 CPUs, 14GB RAM
and 14GB SSD.

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         R: [ '4.3.1' ]
-        os: [ 'macos-latest' ]
+        os: [ 'macos-13' ]
     runs-on: ${{ matrix.os }}
     name: ${{ matrix.R }} ${{ matrix.os }} build
     env: